feat(git): add FetchRemoteBranch, Checkout, ListRemoteBranches to Runner

internal/infra/git/branch.go

@@ -0,0 +1,64 @@
+package git
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"strings"
+)
+
+// FetchRemoteBranch fetches a single remote branch (origin/<branch>).
+// Lighter than Fetch --all when we only need one branch.
+func (r *DefaultRunner) FetchRemoteBranch(ctx context.Context, dir, branch string, cred *Credential) error {
+	ctx, cancel := withCtx(ctx, r.cfg.FetchTimeout)
+	defer cancel()
+	env, cleanup, err := credentialEnv(r.cfg.TmpDir, cred)
+	if err != nil {
+		return err
+	}
+	defer cleanup()
+	_, _, err = r.exec(ctx, dir, env, "fetch", "origin", branch)
+	return err
+}
+
+// Checkout switches the working tree to the given branch.
+func (r *DefaultRunner) Checkout(ctx context.Context, dir, branch string) error {
+	ctx, cancel := withCtx(ctx, r.cfg.OpsTimeout)
+	defer cancel()
+	_, _, err := r.exec(ctx, dir, nil, "checkout", branch)
+	return err
+}
+
+// ListRemoteBranches queries the remote for all branch names via git ls-remote.
+// Returns branch names with refs/heads/ prefix stripped.
+func (r *DefaultRunner) ListRemoteBranches(ctx context.Context, dir string, cred *Credential) ([]string, error) {
+	ctx, cancel := withCtx(ctx, r.cfg.FetchTimeout)
+	defer cancel()
+	env, cleanup, err := credentialEnv(r.cfg.TmpDir, cred)
+	if err != nil {
+		return nil, err
+	}
+	defer cleanup()
+	out, _, err := r.exec(ctx, dir, env, "ls-remote", "--heads")
+	if err != nil {
+		return nil, err
+	}
+	return parseRemoteBranches(out), nil
+}
+
+// parseRemoteBranches parses git ls-remote --heads output.
+// Each line: "<sha>\trefs/heads/<branch>"
+func parseRemoteBranches(buf []byte) []string {
+	var branches []string
+	sc := bufio.NewScanner(bytes.NewReader(buf))
+	for sc.Scan() {
+		line := sc.Text()
+		if idx := strings.Index(line, "\trefs/heads/"); idx >= 0 {
+			name := line[idx+len("\trefs/heads/"):]
+			if name != "" {
+				branches = append(branches, name)
+			}
+		}
+	}
+	return branches
+}